Buffer Input Imp. Question...RRB...Cornish and Skrydstrup

[jdandry]

So doing some research on buffers and noticed some things,
The Cornish, Skrydstrup and the Voodoo Lab buffers are all 1M input impedance
The RRB is 500k. I thought the lowest you want to go is 1M. Any thoughts?

[BJF]

Hi,

Well there are conventions;) while you can certainly find buffers made for guitar with inputimpedances as high as 10 M Ohm's

A buffer would be a stage that isolates the source from the load per definition and thus makes the source independent of the load.

Looking at how the inputimpedance works as a load on a electric guitar with passive electronics it enters in parallel with the Volume and Tonecontrols in the guitar:

Check out the difference loading a standard Fender Stratocaster and assuming standard wiring:

For neck&middle pick up total load at resonance frequency ( the peak of pick up) and excluding effect of capacitances but at 1M Ohms load 111 K would result while with a load at 500K Ohms total load of 99K would result. However for bridge pick up load would be about 200K Ohms at 1M Ohm load and about 167 K Ohms at 500K load.

So what does this mean?

Well at the resonance of pick up there would be a peak- you can view a pick up as a two pole low pass filter with resonance-now the height of the resonance peak depends on the load and if you'd load at say 10K Ohms on same guitar as in above example you'd dampen the whole resonance and a lowpass filter without resonance would result.

Note that the shape of the peak at resonance is what sets the typical sound of a guitar pick up.

If you place a variable load on a strat type pick up as in the example you can hear how this peak changes with the load.

Note also that since a guitar pick up has a choke output it creates a filter with capacitances in cables connecting to amplifier et.c. and this could lead to the false assumption that a cable has a sound whereas it definitely forms a filter that affect sound by loading the pick up.

Keeping that in mind and putting a buffer in with a very high input impedance such as a say 4 M Ohms and resonance peak will now be high and if you compare that with driving a standard guitar cable and amp input there would most likely be a sharp increase in high frequency response.........

You could tailor the load created by the buffer to make a negliable difference compared to driving 6 Meter cable.

Notice that with the 500K load total load on bridge pick up is lower in value and this will affect the high peak of the bridge pick up slightly in such a way to make the high treble peak be more alike that of the neck and middle pick up allthough this by a small measure while it may be just the small measure that means that you do not have to adjust trebel control on your amp when changing pick up. Wether this change would be heard at all is very much dependent on the remaining system

Or you can just set a standard like this that most tube amps have 1M inputimpedance and are DC coupled meaning no capacitor in line and make your buffer like that.

Read further here:
viewtopic.php?f=3&t=868

viewtopic.php?f=10&t=1762


I will say this I have made many studies on the effects of load on a number of guitar pick ups and also deviced internal loads in guitars to tailor the response of pick ups.

I'd say as a rule you'd find that when the inputimpendance of guitar effect is lower than 180K Ohms you will have frequency effects that may either be wanted like in many fuzzes or that may lower treble in such a manner that is a loss.

I would also just for fun suggest that you build a box with just in- and output jacks and place a variable resistor of 5 M Ohm to ground.
Dial and listen

Oh and then after that one could go into examining the effects of capacitive load.......

Have fun
BJ

[jdandry]

Thanks for all the info Bjorn! I went ahead and bought a Red Ruby, mostly for the boost and added bonus of the buffer.
I also just ordered a Cornish LD-1 buffer/line driver from Pete to use as a stand alone buffer, I couldn't resist!